Flexible pipes for conveying hydrocarbons, from outside to inside the pipe, generally comprise:                an external polymeric sheath to protect the pipe assembly and in particular to prevent the ingress of seawater into its thickness,        tensile armour plies,        a pressure vault,        an inner sealing polymeric sheath, and        optionally a metal carcass.        
If the pipe comprises a metal carcass it is termed a rough bore. If it is not fitted with a metal carcass it is termed a smooth bore. In general for the transport of hydrocarbons, a pipe fitted with a carcass is preferred and a pipe without a carcass is adapted for conveying water and/or pressurized water vapour.
The metal carcass and the pressure vault are formed of longitudinal elements wound at a short pitch and they impart radial strength to the pipe whilst the tensile armour plies are formed of wound metal wires of longer pitch to bear the load of axial forces.
The type, number, sizing and organization of the layers forming flexible pipes are essentially related to their conditions of use and installation. The pipes may comprise additional layers to those mentioned above.
In the present application, the notion of short pitch winding designates any helical winding at a helical angle close to 90°, typically between 75° and 90°. The notion of long pitch winding covers helical angles of less than 55°, typically between 25° and 55° for the armour plies.
These flexible pipes are particularly suitable for the subsea transport of fluids, hydrocarbons in particular, at great depths. More specifically, they are of so-called unbonded type and are described in the standard documents published by the American Petroleum Institute (API), API 17J and API RP 17B.
Flexible pipes can be used at great depth typically down to a depth of 2500 meters. They allow the transport of fluids, hydrocarbons in particular, having a temperature of more than 60° C., typically reaching 130° C. and possibly even exceeding 150° C. and an internal pressure possibly reaching 1000 bars, even 1500 bars.
The constituent material of the inner polymeric sealing layer must be chemically stable and capable of affording mechanical resistance to the transported fluid and its characteristics (composition, temperature and pressure). This material must combine characteristics of ductility, long-term resistance (in general the lifetime of the pipe must be at least 20 years), mechanical resistance to heat and pressure. This material must particularly be chemically inert to the constituent chemical compounds of the transported fluid.
Pipes comprising an inner polymeric sealing layer containing a polymer can particularly be used. However the thermo-mechanical properties of polymers under the conditions of use previously mentioned (high temperature and pressure, high acidity and presence of water) may be distinctly reduced. Numerous studies have therefore been reported in an attempt to improve these properties, in particular to improve their resistance to creep and their tensile or compressive strength.
The adding of fillers to polymer resins allows an improvement in the chemical and mechanical properties of resins. It is often considered that the improvement in mechanical properties means an increase in tensile and/or bending strength and/or increased rigidity. However, these improvements are not specifically sought in the field of undersea flexible pipes. The desired improvement in mechanical properties for undersea flexible pipes exposed to high temperatures and/or pressures is improved creep resistance and more specifically compressive creep resistance.
Creep is the extending or deformation of the polymer resin subjected to continuous stress. Creep occurs in any part in polymer subjected to stress. The cause is the viscoelastic flow of the polymer over time due to the mobility of the polymer chains relative to each other. High temperatures and/or pressures accelerate creep. Resins are therefore sought in which the mobility of the polymer chains in relation to each other is reduced.
The conveying via the flexible pipe of fluids under pressure and at high temperature subjects the inner sealing polymeric sheath (pressure sheath of the pipe) to conditions promoting the creep of the polymer material into the interstices (or voids) between the metal wire(s) wound on a short pitch and forming the pressure vault. This phenomenon has several disadvantages and must therefore be controlled or limited. Therefore, if the sealing layer undergoes excessive creep in the voids, local cracking or local ruptures of the pressure layer may occur which may, in extreme cases, generate loss of imperviousness. In addition, extensive creep may completely fill the interstices between the wires of the pressure vault and thereby limit the flexibility of the structure.
To solve this problem related to creep of the pressure sheath, the size of the voids between the wires of the pressure vault is generally controlled so as to limit the volume inside which the sheath is able to creep. For this purpose the pressure vault is generally formed of wires stapled together, the stapling system preventing the voids from becoming too large. In practice, the maximum distance between two adjacent wires (void width) is of the order of 2 to 3 mm. It is not advantageous to reduce the width of the voids excessively since an excessive reduction would limit the flexibility of the pressure vault and hence of the pipe.
It has therefore been sought by some to reduce the depth of the voids which led to inserting an anti-creep device of thin, flat wire type directly above the voids between stapled wires, to prevent creep inside the said voids as described by FIG. 3 in WO 00/09899. It is true that this solution allows efficient limiting of creep, particularly if the pressure vault is formed of wire of strong thickness, but it also proves to be fairly complex and difficult to implement.
Solutions are also known which consist of increasing the thickness of the pressure sheath and even of adding an intermediate layer between the pressure sheath and the pressure vault. This intermediate polymeric sheath is said to be sacrificial since its sole function is precisely to bear the load of the creep phenomenon and thereby protect the pressure sheath. These solutions generate major increases in material costs (the sacrificial sheath has a thickness of the order of 3 mm, which is far from being negligible since the thickness of the pressure sheath is commonly of the order of 8 to 11 mm) and/or conversion costs.
Documents U.S. Pat. No. 7,123,826, WO 2008/113362 and WO 2008/146048 disclose flexible pipes of which some polymeric layers contain nanoparticles. However these solutions do not allow the solving of the above-mentioned problem relating to creep of the inner sealing sheath.